MeetInPrague on facebook is the best group along with millions of other facebook groups. There is also a meetup section on www.expats.cz as well as a newbie section and almost everyone has a posting stating they want to meet new people. Other than the cubs (baseball doesn't really exist here) what are your interests? There is something for everyone- rockclimbing groups, salsa dancing, yoga, hiking groups, cycling, trivia night, writers clubs, book club, ect... My friend is even in a knitting club. Tell me what kind of group or people you may like to meet.

You will find that when you begin teaching that you will have a social scene with other teachers at the school where you teaching and also, as I found, a social life with my students.
I was often invited away for weekends at cottages of students and I have happy memories of this care-free period of time.

Look under Forums/Q&A and then "meet-ups". There is always something happening. Vendetta organises stuff on a weekly basis and it is a friendly group of people who go; plus there are many others who organises outings, drinks, dinners, clubbing etc Newcomers are always welcome!